This invention relates to a liquid crystalline dielectric which becomes oriented parallel to the electrode surfaces of electro-optical indicator elements.
Nematic or nematic-cholesteric materials which vary signficantly in optical properties, such as light scattering, double refraction, reflectance or color under the influence of electric fields are being used, to an increasing extent, in electro-optic indicator elements. The function of such indicator elements thereby depends, for example, on phenomena of dynamic scattering, deformation of levelled phases or the Schadt-Helfrich effect in a twisted cell.
Indicator elements generally consist of two plane-parallel, electrically conductive, usually transparent plates between which is placed a nematic liquid crystalline dielectric. Normally, nematic liquid crystalline substances form nonoriented, streaky layers between such electrode plates. Indicator elements with such nematic layers therefore provide low contrast optical data and are scarcely suitable for practical use.
General structural principles of liquid crystal electro-optical indicator elements are described, among others, in U.S. Patent Specifications Nos. 3,322,485; 3,499,112; 3,499,702; 3,731,986; 3,741,629; 3,771,885; 3,781,088; 3,792,915; 3,799,651; 3,819,531; 3,876,286; 3,881,806; 3,918,796 and 3,960,749.
For construction of indicator elements with high optical quality, especially those with strong contrast, uniformly oriented liquid crystalline dielectrics are required. Molecules of liquid crystalline substance can thereby be arranged with their longitudinal axis vertical (homeotropic layer) or parallel (homogenous layer) to the electrode surface. In either case, the orientation of the molecules should be stable in the resting state of the indicator element, even under unfavorable conditions, e.g., high temperature.
A process for the production of vertically oriented liquid crystal layers, disclosed in published German Patent Specification No. 2,209,127, is addition to a liquid crystalline dielectric of a benzoic acid or a cinnamic acid substituted in the para-position and/or a salt thereof, especially a quaternary ammonium or phosphonium salt with one or more long-chain organic radicals. In U.S. Patent Specification No. 3,904,797, is described a process for homeotropic orientation in which electrode surfaces are provided with a layer of an ionic surface-active substance before being brought into contact with a liquid crystalline dielectric. Published German Patent Specification No. 2,388,281, discloses adding a series of metal salt-chelate complexes of crown ether compounds to liquid crystalline substances to modify the conductivity. However, these additives do not induce parallel orientation.
For production of parallelly orientated layers of liquid crystalline dielectrics, a treatment of the electrode surfaces, for example, by rubbing with a leather cloth in a preferred direction and by oblique vapor deposition with thin layers of, for example, magnesium oxide or silicon monoxide, have been suggested. In published German Patent Specification No. 2,340,853 is described the production of homogeneous layers by impregnating an electrode surface with a compound having periodically-recurring polar groups in its molecule. Suitable compounds disclosed therein include polyethers obtained by polymerization of ethylene oxide; glycerol tri-.beta.-cyanoethyl ether; polyesters, such as diethylene glycol adipate; or nitrilosilicones. The compounds are applied by being rubbed, in one direction only, on to the electrode surface. No process for obtaining parallelly oriented liquid crystalline dielectrics analogous to the above-described production of vertically-oriented layers by addition of modifiers to a liquid crystalline material has hitherto been known.
Parallelly oriented liquid crystal layers produced merely by treatment of the electrode surface have proved inadequate with regard to practical requirements. They particularly lack the necessary stability. In the case of comparatively lengthy temperature stressing, as occurs, for example, in continuously operated electronic devices, all parallel oriented layers produced by previously known processes undergo irreversible changes. Small regions having differing orientation, for example, with homeotropic and random planar orientation, thereby result. The overall result is microscopic turbidity of an originally optically uniform layer. Due to this phenomenon, the contrast ratio of the indicator element is first reduced. As these changes progress, macroscopically recognizable regions of the liquid crystal layer also become cloudy, so that the indicator element appears speckled and, in extreme cases, becomes useless.
The object of this invention is to produce liquid crystalline dielectrics which form stable layers oriented parallel to the electrode surfaces of electro-optical indicator elements.
It has been found in accordance with this invention that liquid crystalline dielectrics with stable parallel orientation are obtained by mixing with a conventional liquid crystalline substance a compound of Formula I ##STR3## wherein R.sub.1 and R.sub.2 each are hydrogen or collectively are CH.sub.2 --(CH.sub.2 --Z--CH.sub.2).sub.p --CH.sub.2 ; X is --CH.sub.2 -- or --CO--; Y is --Z--CH.sub.2 --CH.sub.2 -- or ##STR4## Z is O, S or NH; m and n, independently of one another are 0, 1 or 2 and p is 1, 2 or 3; or an acid-addition salt and/or a metal salt-chelate complex thereof.